Apple's MacOS Sierra is version 10.12 of the Mac operating system released in October 2016. MacOS Sierra is a modern operating system that combines the power and stability of UNIX with the legendary elegance of the Macintosh.
Download MacOS Sierra from Apple (within App Store)
macOS Sierra: Upgrade Checklist
MacOS Sierra: Technical Specifications
MacOS Sierra: Support
MacOS Sierra: Communities
Referenced from: Apple.com